1. Field of the Invention
The present invention relates to an image forming apparatus utilized for electrostatic recording, electro-static printing, etc., on an electrophotographic type recording apparatus, etc.
2. Description of the Related Art
In an image forming apparatus for forming a toner image, by a predetermined processing, on a recording sheet or other transfer media, in an electrophotographic type recording apparatus represented by, for example, a facsimile machine, a photosensitive member on the surface of a photosensitive drum contained in the apparatus encounters a repeated electrostatic image forming action and mechanical stress by a cleaning blade, etc., accompanied by a repeated cleaning action. This photosensitive member is normally shorter in service life than other component parts and, from the standpoint of operation, it becomes necessary to replace it with a new photosensitive member after a predetermined life limit, such as a service life, is reached.
For example, JP3-56474 (CANON case), JP1-41985 (CANON case) and JP1-41986 (CANON case) are known as the existing documents associated with the technique as set out above and have the following problems.
The electrophotographic type recording apparatus of today is often of such a type that the user can readily replace the photosensitive member with a new one and that, for ease in maintenance, the photosensitive drum together with the associated disposable component parts such as a charger, developing unit and cleaner, are provided as a combined unit (i.e. a process unit) in the apparatus. Further, these disposable component parts together with the photosensitive member are often of such an integral type that they can be replaced all at a time with a new unit. This integral unit enables the user to replace the disposable component parts with new ones by replacing the process unit. This ensures a readier maintenance operation. A toner is sealed in a hopper for holding it for development (i.e. a toner sealing type). Further, since a spent toner produced at the cleaning operation by a cleaner is held in its own container, the user can readily handle the toner.
In the toner-sealed type process unit as set out above, an amount of toner to be used is restricted to a given extent and, if a black area, i.e., an image area, on a recording sheet is greater, upon recording, than a white background of the sheet, that is, if a black rate is higher on the sheet, the toner is earlier consumed to an empty level before the photosensitive member reaches its own predetermined service life limit, thus substantially hastening the replacement of the process unit as a whole. Therefore, the photosensitive member still retaining its own effective service life has to be thrown away by the disposal of the process unit and hence the running cost on the apparatus is increased, resulting in an economical disadvantage.
In order to eliminate such a disadvantage, an addition arrangement, for example, is utilized to enable an additional supply of toner to be made to a hopper for holding a fresh toner therein, so that the supply of the toner can be continued until the photosensitive member finally reaches its own service life limit. By so doing it is possible to secure an optimal service life of the process unit. In the above described type of process unit, the replacement time of the process unit is set based on a given cumulative value corresponding to the number of printing cycles of the photosensitive member. Put in another way, the suitable replacement time of the process unit is determined to be in order when a predetermined number of sheets are printed. In the case where the amount of toner reaches a deficient level before a given printing number, it is possible for the user to make an additional supply of toner to the hopper as desired. This type of apparatus enables the user to utilize the process unit to a maximum possible limit of use until the photosensitive member reaches its own available service life limit.
Since, in this case, a spent toner holding section for collecting a spent toner is constant in its capacity, that type of apparatus involves the following disadvantage. Generally an amount of spent toner is increased in proportion to the amount of fresh toner supplied and, when a higher black rate is involved, more fresh toner is used to form images on the sheet. If such a higher black rate occurs many times upon printing, the spent toner holding section is nearly overflowed with the spent toner until the photosensitive member reaches its own given life limit. As a result, the spent toner is overflowed out of the spent toner holding section or clogged at some places on a drive system, there being a risk that the drive system is locked or fails. It may be considered that, in order to avoid such a problem, an overflow sensor must be provided so as to inform the user that the spent toner holding section is nearly overflowed with the spent toner or the spent toner holding section is made somewhat larger than normal while leaving some margin in the spent toner holding section. The spent toner holding section having such a large margin makes the process unit bigger. And in the case a sensor for detecting the filling of the spent toner is arranged, when the images are recorded many times on the recording sheet at a lower black rate, that is, at a higher white rate, the photosensitive member reaches its own given life limit before the spent toner holding section is detected to be filled with the spent toner.
In the aforementioned apparatus, since the life limit (i.e., a service life), of the process unit per se is set in accordance with a given service life limit period of the photosensitive member, if an image is often printed at a higher black rate against a white background on the sheet, there is a possibility that the spent toner holding section will be fully filled with the spent toner before the process unit reaches its own life limit. It is considered that, in order to prevent such a situation, an overflow sensor is provided on the spent toner holding section or the spent toner holding section is made somewhat large. Upon frequent printing of images at a high white rate on the sheet a problem has been encountered that the photosensitive member of the process unit reaches its own life limit before the spent toner holding section is detected to be fully filled with the spent toner. Therefore, the above-described situation, is impractical.